Maintaining a specimen within a field of view when switching or translating objectives may be affected by the degree of parcentricity. A specimen may shift from a field of view when, for example, objective lenses are translated across their optical axes. Switching or translating objective lenses can create deviations resulting in a specimen being lost from a field of view when, for example, a greater magnification objective is selected and translated. Objective lenses may be switched manually or automatically and are often translated along a single degree of freedom, such as along a straight line or an arc. Subsequently, maintaining parcentricity while translating objectives is problematic.
Conventional objectives have several problems with regard to parcentricity. For example, translating an objective to a higher power magnification objective typically shifts the center of the field of view. Such shifts can also cause users to incur significant time and effort attempting to fix and align an objective after translation. Current methods of correction for parcentricity during manufacturing require specially-trained technicians. Furthermore, the parcentricity error can grow due to mechanical wear—restoration of parcentricity requires service by technicians. Conventional objectives typically provide for only a single degree of freedom during operation, rendering correction of parcentricity errors impossible during operation. Alignment methods for parcentricity are available during manufacture, but they are not appropriate during operation. These alignment methods employ translation of an objective across its optical axis. These parcentricity methods are typically not performed by an operator of the microscope. Alignment of parcentricity requires considerable effort by a specially-trained technician.
Conventional methods for beam steering can be applied to the exiting light of an objective to achieve parcentricity but these existing methods are impractical. Specifically, a pair of Risley prisms can establish parcentricity, but this alignment method would not be appropriate for an ordinary operator. A Risley prism is a wedge of glass with a freedom of rotation about a normal to one of the faces of the prism. Alignment of Risley prisms also require a specially-trained technician. A pair of Risley prisms are impractical when applied to parcentricity of microscope objectives.
A pair of Risley prisms is often used to provide two degrees of freedom in beam steering applications. Risley prisms deflect rays in a range of deflection that is typically less than 5°. However, Risley prisms cannot be used to achieve a net deflection of zero due to finite differences between the wedge angles of the prisms. The finite difference between wedge angles creates a small circle of exclusion, resulting in a range of deflection in the shape of an annulus. This circle of exclusion is problematic in application to parcentricity. Further, when used with converging rays, Risley prisms create aberrations in the form of astigmatisms that are dependent upon the variable orientation of the prisms. When used with parallel rays of different colors conventional Risley prisms create aberrations known as lateral color.